Darlington pair transistor amplifiers are often used in applications needing large gains. A pair of transistor elements are cascaded to create an amplifier with a gain factor that is about equal to the product of the gain factors of each of the transistor elements. FIG. 1 shows a traditional Darlington pair amplifier 10, according to the prior art. A first NPN bipolar transistor element Q1 and a second NPN bipolar transistor element Q2 are cascaded to create a Darlington pair. The first NPN bipolar transistor element Q1 is an output transistor element and the second NPN bipolar transistor element Q2 is a driver transistor element that feeds the output transistor element. The collector of the first NPN bipolar transistor element Q1 provides an RF output signal RFOUT. A first resistive element R1 is coupled between the emitter of the first NPN bipolar transistor element Q1 and ground. The base of the first NPN bipolar transistor element Q1 is coupled to the emitter of the second NPN bipolar transistor element Q2. A second resistive element R2 is coupled between the emitter of the second NPN bipolar transistor element Q2 and ground. The collector of the second NPN bipolar transistor element Q2 is coupled to the collector of the first NPN bipolar transistor element Q1. An RF input signal RFIN feeds the base of the second NPN bipolar transistor element Q2. A third resistive element R3 is coupled between the base of the second NPN bipolar transistor element Q2 and ground. A fourth resistive element R4 is coupled between the base of the second NPN bipolar transistor element Q2 and the collector of the second NPN bipolar transistor element Q2.
The third and fourth resistive elements R3, R4 may provide feedback, biasing, or both, for the traditional Darlington pair amplifier 10. Power for the traditional Darlington pair amplifier 10 is provided by external components, which are coupled to the collectors of the first and second NPN bipolar transistor elements Q1, Q2. A traditional bias resistor R5 and a first inductive element L1 are coupled in series between a direct current (DC) supply VDCSUPPLY and the collectors of the first and second NPN bipolar transistor elements Q1, Q2. The traditional bias resistor R5 is used to regulate bias current in the traditional Darlington pair amplifier 10. Without the traditional bias resistor R5, the bias current may vary greatly over temperature, with manufacturing process variations, or both. However, bias current through the traditional bias resistor R5 wastes power and lowers the operating voltage available to the traditional Darlington pair amplifier 10. The lowered operating voltage may reduce efficiency, may lower a 1 decibel (dB) compression point (P1dB), may lower an output third-order intercept point (OIP3), or any combination thereof, which may reduce the effectiveness of the traditional Darlington pair amplifier 10. Reducing the value of the traditional bias resistor R5 may restore some of the operating voltage; however, the bias current variations may be unacceptable.
Some applications require a Darlington pair amplifier that can be powered from a five volt or a three volt DC supply VDCSUPPLY. Thus, there is a need for a Darlington pair amplifier that can operate without a traditional bias resistor R5 or operate with a reduced value traditional bias resistor R5, can operate properly with a five volt DC supply VDCSUPPLY, a three volt DC supply VDCSUPPLY, or both.